1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device in which occurrence of a moiré fringe is suppressed and display nonuniformity produced by a moiré fringe is accordingly decreased so that display quality is improved.
2. Description of the Prior Art
In recent years, notebook type personal computers are becoming remarkably low-profile and lightweight so that they can be carried easily. Accordingly, liquid crystal display devices are also becoming low-profile and lightweight. Making low-profile a transmission liquid crystal display device having a backlight amounts to making the backlight low-profile. Components most influenced by rendering the backlight low-profile include a light guide plate. Making the light guide plate low-profile narrows its light-incident end face and decreases the incident light amount, leading to a decrease in luminance. Hence, a measure for outputting incident light to a liquid crystal display panel efficiently is necessary. Therefore, a lens sheet obtained by forming a group of small prisms on one surface of a sheet to provide a light condensing effect is often used.
In order to improve the luminance of the backlight, a liquid crystal display device obtained by stacking two lens sheets to enhance the light condensing effect is available.
FIG. 1 is a sectional view showing the arrangement of a conventional liquid crystal display device described above in which two lens sheets are stacked. This liquid crystal display device is comprised of a liquid crystal display panel 5, backlight 6, driver IC (not shown) for driving the liquid crystal display panel 5, and signal control circuit (not shown) for the driver IC. The liquid crystal display panel 5 is comprised of upper and lower electrode substrates 2 and 3 arranged adjacent to each other, and upper and lower polarizing plates 1 and 4 respectively adhered to the outer surfaces of the upper and lower electrode substrates 2 and 3. The backlight 6 is comprised of a fluorescent tube 13, reflector 12, light guide plate 10, reflecting sheet 11, diffusing sheet 9, upper lens sheet 7, and lower lens sheet 8. The outer surface of the upper polarizing plate 1 is roughened to form a roughened surface 14 for antiglare. In this prior art, a group of substantially parallel ring-like curves (fringes) form to produce display nonuniformity, impairing the display quality. According to an analysis of the cause of the fringes, two lattice fringe images having an equal grating constant are superposed on each other to form an interference fringe that appears as a “moiré fringe”. A moiré fringe is an interference fringe generated when two gratings having an equal grating constant are stacked on each other at a small angle. The grating that produces the moiré fringe results from the surface shape of a lens sheet.
The moiré fringe described above will be explained.
FIGS. 2A and 2B show how a lens sheet generates grating fringes. As shown in FIG. 2B, prisms are formed on a lens sheet 20. Depending on the angle of incidence of light, light incident on the lens sheet 20 is reflected by the slant of a prism to have a directivity. Thus, as shown in FIG. 2A, a grating 18 in which bright portions formed by reflected light beams 19a and dark portions where reflected light beams 19b are not present is generated.
FIGS. 3A and 3B show the principle of generation of a moiré fringe by lens sheets. As shown in FIG. 3B, grating fringes (real-image grating) 21 generated in the upper lens sheet 7 include two reflection paths. According to one path, light is directly reflected by the upper lens sheet 7. According to the other path, light is transmitted through the upper lens sheet 7 and is reflected by the lower lens sheet 8. The grating fringes generated by the former path correspond to a grating directly generated in the upper lens sheet 7, which is a real-image grating 21. The grating fringes generated by the latter path correspond to a reflected image of the real-image grating 21, which is a virtual-image grating 22. When the real-image grating 21 and virtual-image grating 22 are superposed on each other at a small angle, as shown in FIG. 3A, a moiré fringe 23 is formed. In practice, a displacement of small angle is produced by deflection or the like of the lens sheets. The lines of one grating overlap those of the other grating. A line obtained by connecting points where these lines intersect each other has a higher contrast than that of the original lines of the gratings, so that it is visually recognized as a moiré fringe. The moiré fringe is traced back to the gratings that occur due to the surface shapes of the lens sheets. Hence, the moiré fringe can be eliminated only by using lens sheets which have a surface shape other than a prism type shape and which provide a light condensing effect equivalent to or better than that of prism type lens sheets.
Other than a prism type lens sheet, however, no lens sheet is available which has a high light condensing effect and can be available at a low cost almost equal to that of the prism type lens sheet. If an optical member such as a diffusing sheet is further used to suppress the moiré fringe, the thickness of the liquid crystal display device increases, leading to an additional increase in cost for this optical member. A means must therefore be taken to prevent the moiré fringe without impairing the low-profile shape.
In view of the above problems, various conventional examples as follows are proposed.
As the first conventional example, a liquid crystal display device is described in Japanese Unexamined Patent Publication No. 04-175727. According to this device, the outer surfaces of polarizing plates on the upper and lower surfaces of a liquid crystal display element are antiglare-processed. This prevents occurrence of Newton rings (interference fringes). Although not specifically described, the antiglare process probably means formation of three-dimensional shapes.
As the second conventional example, a liquid crystal display device is described in Japanese Unexamined Patent Publication No. 05-053103. According to this device, the lower surface of a lower polarizing plate is antiglare-processed to provide a polarizing plate which is not easily damaged. An upper polarizing plate is also antiglare-processed, in the same manner as in the prior art.
As the third conventional example, a liquid crystal television is described in Japanese Unexamined Utility Model Publication No. 62-193224. According to this television, anti-reflecting coatings are formed in tight contact with upper and lower polarizing plates.
As the fourth conventional example, a liquid crystal display device is described in Japanese Unexamined Patent Publication No. 01-234822. According to this device, a light scattering surface is formed on the lower surface of a lower polarizing plate. This aims at removing and decreasing reflection by the surface of a television. The light scattering surface forms a roughened surface having small three-dimensional shapes. This device also aims at eliminating interference of light emerging from a backlight.
As the fifth conventional example, a liquid crystal display device is described in Japanese Unexamined Patent Publication No. 62-199722. According to this device, the outer surface of a polarizing plate on the rear side forms a roughened surface. This invention aims at making unnecessary a rear reflecting plate (ordinarily called a diffusing plate) and furthermore a light guide plate.
As the sixth conventional example, a liquid crystal display device and a polarizing film used by it are described in Japanese Unexamined Patent Publication No. 06-034961. A light diffusing layer is formed on the surface of a polarizing film on the rear side of the liquid crystal panel of this liquid crystal display device. The light diffusing layer has small three-dimensional shapes (small embosses). This invention aims at making a light guide plate unnecessary.
As the seventh conventional example, a laminated body is described in Japanese Unexamined Patent Publication No. 03-120037. In this laminated body, small three-dimensional shapes are formed by coating on the surface of one or each of films to be laminated, so that interference fringes do not stand out when the films are merely stacked, not adhered to each other.
The conventional examples described above have the following problems.
In the first conventional example, although Newton ring interference fringes may be suppressed, a moiré fringe cannot be suppressed. This is obvious because a moiré fringe is recognized even when the roughnesses of the roughened surfaces of the polarizing plates are increased. A moiré fringe is caused by grating fringes resulting from the surface shapes of prism type lens sheets. To suppress the moiré fringe, the occurrence of the grating fringes themselves must be suppressed, or the grating fringes must be scattered by roughened surfaces so that they become thin. In the first conventional example, use of lens sheets is not anticipated, and only the antiglare process for the polarizing plates is explained. In order to eliminate a moiré fringe, however, the surface roughening process for the lens sheet is also significant. If the surface roughnesses are increased so that the grating fringes become thin with only the antiglare process for the polarizing plates, light from the backlight is scattered by the roughened surfaces to decrease the luminance, which is another problem.
In the second conventional example, the influence applied by the surface roughness of a roughened surface to the optical characteristics such as luminance is not described. This is because the use of a prism type lens sheet is used in order to realize low-profile, high-luminance liquid crystal display device is not contemplated, and roughening the lens sheet in order to suppress a moiré fringe generated by the lens sheet is not an option here.
The third conventional example does not contribute to making a liquid crystal display device low-profile, and the cost is increased. This is because since anti-reflecting coating substrates are prepared separately of the polarizing plates, the thickness of the device increases by the thickness of the anti-reflecting coating substrates. Also, an extra cost is required by the anti-reflecting coating substrates.
In the fourth conventional example, although interference fringes may be suppressed, it does not contribute to making the liquid crystal display device low-profile. This is because since a diffusing plate is arranged on that surface of the backlight which opposes the liquid crystal display panel, the thickness of the device increases by the thickness of the diffusing plate.
In the fifth conventional example, the influence of the surface roughness of the roughened surface on the optical characteristics such as luminance is not described, in the same manner as in the second conventional example. This is because the use of a prism type lens sheet in order to realize low-profile, high-luminance liquid crystal display device is not anticipated, and roughening the lens sheet in order to suppress a moiré fringe caused by the lens sheet is not an option here.
In the sixth conventional example, if the light scattering layer is merely provided, the luminance uniformity suffers. This is because the light transmission characteristics of the light scattering layer of the polarizing film proposed in this conventional example are those obtained when an incident light beam becomes incident in the same direction as that of the normal to the light scattering layer. In the liquid crystal display device using the polarizing film, since a light source is disposed on the end portion of the device, light from the light source strikes the liquid crystal display device obliquely and is specularly reflected by the reflecting plate. The incoming light on the light scattering layer of the polarizing plate accordingly strikes in an oblique direction and not in the direction of the normal to the light scattering layer. Hence, the light transmission characteristics become different from those obtained when light becomes incident in the direction of normal to the light scattering layer. Since the angle of light specularly reflected by the reflecting plate differs between a position close to the light source and a position far from it, the light transmission characteristics change within the display surface of the liquid crystal display device, and the luminance does not become constant.
In the seventh conventional example, formation of roughened surfaces on articles to be stacked can undesirably decrease the luminance. This is because of if a roughened surface is formed on each of particles to be stacked, light from the backlight may be scattered when transmitting through the articles, and its transmission light amount may decrease. Nevertheless, the relationship between the roughness of the roughened surface of each of the particles to be stacked and the luminance is not considered.